Spark plug



March 27, 1945. F. JONES 2,372,429

SPARK PLUG Filed Oct. 51, 1942 Zmnentor Loren E'Jones 5,, CK) M Gttorneg Patented Mar. 27, 194

Loren F. Jones, Philadelphia, 2a., assig'nor to Radio Corporation of America, a corporation of Delaware Application October :1, 1943, Serial No. 464,005 5 Claims. (Cl. 315-39) This invention relates to an improved spark plug for use with internal combustion engines of the type (e. g. aircraft motors) employing high frequency (microwave) ignition systems.

It has heretofore been proposed (see copending application Ser. No. 445,046 to Francis X. Rottenmeyer, filed May 29, 1942) to provide an aircraft motor with an ignition system employing ignition voltages of a frequency considerably above the frequencies normally used for radio communication purposes. Such ignition systems possess many advantages over conventional low frequency ignition systems. Among these advantages are: (a) satisfactory operation is maintained at high altitudes, (b) interference with radio communication is obviated without the use of cumbersome and expensive shielding systems and (c) spark-over caused by vapor condensation, low atmospheric pressure, or carbonization of insulation is veliminated.

The spark plugs employed in such high frequency ignition systems have heretofore included either a transformer adjacent to the plug or have celain bushings usually employed are poor heat conductors and difliculties may arise due to-overheating of the inner electrode of the plug adjacent to the voltage loop or spark-gap.

Accordingly, the principal object of the present invention is to provide an improved spark plug of the general character described and one wherein means are provided for dissipating excess heat from the inner or high potential electrode.

This principal objectis achieved in accordance with theinvention in a microwave spark plug of the type comprising a cylindrical outer electrode and an inner electrode by the provision of a heat conductive connection between said electrode and the motor block (and hence the cooling system) ata point removed from the spark-gap or "voltage loop" a distance. which may be described as corres onding substantially to the electrical equivalent'of from one-eighth to m'quarterwave leingtl. of the particular ignition frequency em p oye Another and important object oi the invention is to achieve the foregoing object in an electrically ellicient and mechanically compact spark plug structure and one which may be accommodated in plug seat or bore of standard design.

The latter object is achieved by the provision on the outer surface of the plug of a bushing of a novel construction but having a thread of'standard pitch for seating the plug in the motor block, and the several parts of the plug are so designed. positioned and arranged, in accordance with the principle of the invention, that the high potential inner electrode is i heat transfer relation with the motor block. an hence with the cooling system of the engine, through the said bushing.

In Fig. 1, l designates the inner and 3 the outer conductor of a low impedance transmission line or coaxial cable which will be understood to be connected atone endvto a source of high frequency voltages, not shown, and at its opposite end to a spark plug 5 which is constructed in accordance with the principle of the present in-- vention. This spark plug 5 comprises a hollow cylindrical outer electrode 1, a; concentric rod-like inner electrode 9, and a gas-tight mica, porcelain or other suitable insulating bushing ll mounted within th bore of the outer electrode and through which the inner electrode extends to form a spark gap l3 at a "voltage loop adjacent tothe lower or inner terminal of the plug. The plug 5 has a circumferential metal flange or nut It on its outer surface adjacent to its upper end and this nut I5 is provided on its lower surface with a dependent circular metal bushing l'l which is spaced from the cylindrical electrode I, and is threaded on its outer surface for engagement with complementary threads in the wall of the motor block M. It will be observed upon inspection of the drawing that there are'a number (in this case, two) of oppositely located apertures la in the wall of the outer cylindrical electrode 1 intermediate its ends and that there is a rod or metal strip 19 which extends at a right angle from the inner electrode 9 through these apertures 7a to the circular threaded bushing 11. This metal connection I9 is provided, in accordance with the invention, for the purpose of main- 'taining the inner high potential electrode 9 in heat transfer relation with the threaded metal bushing l1. and hence with the cooling system of the en ine which in the instant case is shown as comprisin a water jacket J but which may compr air c oled fins. or the like, not shown.

Since the outer cylindrical electrode 1 is connectedthroughthe nut l5, metal bushing-l1 and metallic member I! to the high potential electrode the line.)

8 it is important that this connection present the proper impedance to the ignition voltage at its point (l9a) of connection to the said inner electrode, 9. This is achieved by so matching the impedance of the said heat conductive connection with the impedance of the inner conductor (from the point Isa to the spark-gap l3) that'standing waves are minimized in the transmission line above the point Illa.

The exact physical length of the plug between the spark-gap l3 and the point Illa at which the heat conductive strip I9 is connected to the inner conductor 9 may be somewhat less than onequarter of the wave length of the ignition fre-' What a claimed is: 1; A spark plug for use with an internal combustion engine employing a microwave ignition system, said spark plug comprising inner and outer electrodes terminating in a spark-gap, a metal bushing on said outer electrode for seating said plug in a bore in the cylinder head of said engine, and a heat conductive member connected between said inner electrode and said metal bushing within said cylinder.

2. A spark plug for use with an internal combustion engine employing a high frequency igniquency (it may be as small as one-eighth of saicflfi wave length) as determined by the ratio of the surge impedance of the transmission line to the impedance of the spark-gap is when a spark ,is present. Irrespective (within the indicated limits) of the exact distance between the gap 13 and the point [9a itis important that the impedance and hence the physical length of the heat conductive connection (as measured from the point l9a to the nut [5 on the outer electrode 1) be so chosen that it will balance the reactance (at point Iiia) which obtains by reason of the fact that the length of the line (1 and 9) beneath the point |9a is less than one-quarter of the wave length of the ignition frequency. .(As above indicated, the length of the inner conductor 9 as measured from the gap l3 to the point Isa is made less than one-quarterof the wave length of the ignition frequency because the impedance of the line at the spark-gap I3 is greater, before firing, than the characteristic impedance of In practice, the ideal length of the above described heat conductive conduction (like the length of the inner conductor 9 as measured fromthe spark-gap I: to the point l9a) will ordinarily correspond to a figure within the limits of one-eighth to one-quarter of the wave length of the ignition frequency. Thus, where the wave sions may be multiplied by 21t-1 where n is the multiplier. Thus, wherever in the appended claims reference is made to a dimension or a distance which is substantially the electrical equivalent of a quarter wave of the ignition frequency it will be understood that the said phrase is intended to include not only dimensions within the range of M8 and M4 but also dimensions or distances which are equivalent in electrical effect.

tion system, said spark plug comprising concentric inner and outer electrodes terminating in a spark-gap at a point corresponding to the voltage loop, a metal bushing on said outer electrode for seating said plug in a bore in the cylinder head of said engine and a conductive connection between said inner electrode and said metal bushing within said cylinder at a point removed from said spark-gap a distance which is substantially the electrical equivalent of a quarter-wave length of said high frequency for maintaining said inner electrode in heat transfer relation with said metal bushing. I

3-. The invention as set forth in claim 2. and wherein said heat conductive connection has a length corresponding substantially to the electrical equivalent of a quarter-wave length of said high frequency between its points of connection with said inner and outer electrodes.

-inner electrode and said metal bushing through z 'said aperture.

5. A spark plug for use with an internal combustion engine employing a high frequency lgnition system, said spark plug comprising a hollow cylindrical electrode having an open end, a,

5 concentric inner electrode, a gas-tight insulating bushing in said cylinder through which said inner electrode extends to form a spark-gap at a voltage loop adjacent to said open end of said outer electrode, and a conductive connection between said inner and outer electrodes in the space between said insulating bushing and said voltage loop for dissipating heat from said inner electrode;

LOREN F. JONES. 

